Add incremental context expansion (up/down arrows) to diff viewer

[CaioDGallo]

#enhancement suggestion

When reviewing the current changes, sometimes I need more context of where the change is inserted, maybe knowing which function or class it is actually part of. PostHog Code currently allows for expanding all unchanged lines between two diffs, but sometimes (like the example below), there are a thousand-ish lines (that I am not interested in) between the two changes, breaking the flow and making the user scroll to find the diff they were actually looking at, trying to get more context.

Something like GitHub's arrows that expand file context above and below could be a good fit for this scenario.